/Engine/Plugins/Experimental/MeshModelingToolsetExp/Source/MeshModelingToolsExp/Public/MeshSelectionTool.h
Flip the normals of the selected triangles. This will create hard normals at selection borders.
[UFUNCTION](Programming/UnrealArchitecture/Reference/Functions)(CallInEditor, Category=MeshEdits, Meta=(DisplayName="Flip Normals", DisplayPriority=4)) void FlipNormals()